Plasticity of dopamine (DA) release is important for CNS function and behavior. For example, DA release is depressed in the nucleus accumbens during models of learning and reward, elevated in models of drug dependence, and activity-dependent regulation of DA release appears to mediate the execution of motor tasks and working memory. Most studies of synaptic plasticity in the CNS investigate neurotransmitter systems that provide rapid postsynaptic responses. During the initial period of this PPG, we established techniques that provide the first CNS recordings of presynaptic transmitter release at the quantal level. These electrochemical approaches are used to measure DA, which does not produce rapid postsynaptic currents, and yield the first direct CNS recordings of quantal size and kinetics. Our findings in the initial PPG indicated that the quantal size and quantal frequency of DA release is highly plastic. The mechanisms governing these changes remain mostly unknown. Accordingly, the objective of this proposal is to characterize mechanisms that mediate stimulation-dependent presynaptic plasticity at the level of the synaptic vesicle. These include 1) the expression and modulation of the vesicular fusion pore, 2) modulation of the readily releaseable vesicle pool, and 3) effects of vesicle transporter expression. While these studies can for the most part only be presently conducted in the midbrain DA system, the results may be generally applicable to other CNS synapses.